After your at a point of safely isolating the incoming supply carry on with the rewire, personally I'd get a multi function tester even if it's uncalibrated, it'll still perform all the main safety checks correctly you just won't be able to rely on it's results for disconnection times but you can work them out with a pen and paper the mft results are just to confirm your workings out, someone with calibrated kit can do that at a later date, the mft should still be reliable for ir testing and testing for shorts and connections, you can do them reasonably reliably with a normal multi meter.